Axial flow compressor plant



Patented June 22, 1943 Adolf Baumann,

Wettin'gen, Switzerland, assignor to Aktiengesellschaft Brown Boveri &Cie, Baden, Switzerland, a joint-stock company.

Application January 31, 1939, Serial No. 253,838

In Germany February 1, 1938 I 2 Claims.

This invention relates to axial flow compressors and to mechanisms forregulating the compressor output. and preventing surging.

The characteristic curves for an axial flow compressor are basicallydifierent from the corresponding curves for radial flow compressors andthe methods of regulation that are applicable for radial flowcompressors are not satisfactory in the case of axial flow compressors.

Objects of this invention are to provide axial flow compressors with newmechanisms for regulating the same, the apparatus and methods beingcharacterized by high efficiency and stability of operation at lowcompressor outputs. An object is to provide compressor plants includingan axial flow compressor, an engine or motor for driving the compressorat a substantially con-- stant speed for which the compressor. outputwill satisfy the maximum volume and/or pressure demands, an air or gasturbine, and a manually or automatically controlled valve for divertingto the turbine such fraction of the total amount of air or gas pumped bythe compressor as will result in the desired regulation condition at thepoint of use of the compressed air or gas, i. e. constant pressure,constant volume,'predetermined pressure-volume characteristic, etc.; thepower developed by the turbine being employed to supplement the engineor motor. An object is to provide axial flow compressors that havemechanisms operable to maintain a desired condition of constantpressure, constant volume or predetermined pressure-volume relationshipat the point of use of the compressed'air Or gas when a constant speedmotor drives the compressor at a speed such that the maximum volumedemand is delivered at the required pressure, the regulating mechanismbeing operable to divert through an expansion turbine such fraction ofthe compressor output which will maintain the desired condition.

These and other objects and advantages of the invention will be apparentwhen taken with the accompanying drawing in which:

Fig. 1 is a curve sheet showing the pressurevolume and load-volumecharacteristics for axial flow and radial flow compressors;

Figs. 2 to 5, inclusive, are similar characteristic curves for axialflow compressors regulated in accordance with this invention; and

6 is a diagrammatic illustration of an axial flow compressor plantembodying this invention.

In Fig. 1, the heavy solid line curves La, Pa, show the variation of theload L and' discharge pressure P of an axial flow compressor withchanges in the output or volume V of gas or air delivered by thecompressor, and the heavy dotted line curves, Lr, Pr, show thecorresponding variations'in the case of a radial flow compressor. Thedesigns of the two compressors are assumed to be such that the powerinputs to the compressors are equal when they deliver the same normalvolume Vn of air or gas at the same pressure. This condition isindicated by the crossing of the pressure curves Pa, Pr and the load-curves La, Lr at points I, I", respectively.

'It will be seen that the power input or load Lr of the radialcompressor increases and the pressure Pr decreases with increasingvolume output V, but the load La and pressure Pa for the axial flowcompressor drop rapidly with an increase in the output. The so-calledsurging limit at a decreased output Va of the axial flow compressor ismuch closer to the normal output Vn than is the surging limit volume V1for the radial flow compressor. The significance of thesecharacteristics is that, upon a decrease in output volume, the axialflow compressor reaches an untable condition more rapidly than" does aradial flow compressor, and that the load or power consumption risesrapidly to point Z'a as the pressure rises to point 2a. The operatingpoints 2'1, 21' for a radial flow compressor at the surging limit Vr areindicated on curves Lr, P1, respectively. 7

. One method of obtaining stability in the operation of radial flowcompressors has been toipump the limiting quantity Vr through thecompressor for all lesser consumption demands and to blow off thatquantity of gas or air which was in excess of some small demand volume,for example the volume V3. The load and pressure forthe radial flowcompressor are thus stabilized at the valuesof points 2'1' and 21',respectively. Similar regulation in the case of an axial flow compressorwould require the high power input at level L2 for all low outputvolumes, and would deliver the ccampressed gas or air at the highpressure level P x This increasing pressure at reduced output is usuallyunnecessary and may be undesirable. In accordance with this invention,the normal volume Vn is pumped through the compressor at the pressurelevel P3-l, and the fraction in excess of the required volume V3 isblown off, 'thus holding the power input at the valueL'a when thecompressed gas or air is discharged at atmospheric pressure. This blowoff at the normal discharge pressure insures stable operation but thepower input L'a is substantially above the corresponding power input Irrfor the radial flow compressor at reduced outputs. Increased efllciencyis obtained by discharging the surplus gas or air through an expansionturbine coupled to the compressor, the net power input to the axial flowcompressorunit then droppin along curve L"a as the volume demanddecreases. Line L"a drops below curve Lr and thus indicates that axialflow compressor and expansion turbine system is more eincient than theradial flow compressor at low outputs. The discharge pressure is keptconstant over a range or output demand and it is not necessary toprovide additional regulating members or to alter the rotary speed ofthe compressor to prevent surging.

This type of regulation is particularly useful when the consumptionrequirements are such that the pressure decreases as the volumedecreases, Fig. 2. Assuming that the demand varies between a volume VIat pressure PI and a smaller volume V3 at lower pressure P3, the axialcompressor is driven at such speed that the point 1, corresponding toPI, VI, falls on the pressurevolume curve.- According to the invention,the compressor is operated at the lower pressure level, point 2 on thepressure-volume curve Pa, corresponding to the larger compressor outputV2. The blowing off of air or gas to increase the output from VI to .V2eifects a substantial .decrease in the load as the power input dropsfrom point I' on the load curve La to point 2. The

quantity AV in excess of the demand require:

ment may be expanded in a turbine to supply power for driving thecompressor, and the net power input is thereby reduced still further to.

lationship. In all cases the expansion turbine serves as a regulatingmember for establishing the desired condition and for preventingsurging.

As shown by the curves oi. Fig. 3, the regulated condition may be thatof a constant volume, for example V3, that may normally be supplied at apressure P2 corresponding to operating point 2 on the curve Pa. Theoutput of the compressor at operating point 2 exceeds the demandrequire! ment V3 by a surplus AV that may be discharged through therecovery turbine. The power consumption' when operating at point 2 oncurve Pa is substantially less than that for. operation at point I atwhich the required volume V3 constitutes the total compressor output ata pressure PI. The power consumption at operating point deliverypressureis the maximum volume that can be discharged at that pressure and thesur plus AV or A'V above the actual demand at pressure PI or P3,respectively, is blown off to the expansion turbine through apressure-regulating valve. and the net power input drops with decreasingconsumption along the net load lines L and L respectively, as -theconsumed volume varies between points I and 2 on pressure line PI andpoints 3 and l on pressure line P3.

Another type oi! regulated condition is that oi! simultaneous variationof pressure with changmg volume demand in accordance with a selectedparabolic pressure-volume curve such as indicated by lines A or B ofFig. 5. The particular parabolic curve of operatingconditions is set byhand adjustment and the axial flow compressor is driven at that speedfor which the pressurevolume curve Pa crosses the control condition lineA at operating point I corresponding to maximum compressor output at themaximum demand pressure. As the volume demand decreases, a greater orless surplus volume AV is blown oil? to obtain the correspondingreduction in outlet pressure, and the operating point drops to point 2on curve Pa when the delivery pressure falls to point 3 on curve A. Thecorrespond ing drop inthe load is from point I to point 2' on the loadcurve La when the surplus volume AV is discharged to open air, or frompoint I to point 3 on curve L if the expansion turbine is used. Agreater surplus volume A'V may be available when the control member isset for operation on curve B corresponding to a smaller volumeconsumption.

' the speed regulating methods without requiring I is indicated by pointI' of curve La, and the power drops to point 2' on curve La if thesurplus volume AV is exhausted to atmosphere or to point 3' of curveLa-- if the surplus is discharged through the turbine. Any desiredvolume, for example volume V4, may be set in the usual way by adjustingthe regulating member by hand. The surplus A'V may be employed in theturbine to reduce the power input to point 4', and surging is preventedat all reduced demand volumes as the same volume V2 is discharged by thecompressor regardless of the lesser volume that is to be supplied to thepoint of use.

Similarly, as shown-by the curves of Fig. 4, the regulated condition maybe that of constant pressure PI or P3 corresponding to operating pointsauxiliary surging gears to insure stable operation; the compressor atall times delivering the full volume along the pressure-volumecharacteristic and any surplus being delivered to the expansion turbineto reduce the power consumption.

A typical assembly of a compressor and regu-.

lating equipment, as shown diagrammatically in Fig. 6, includes theaxial flow compressor C and expansion turbine T on the same shaft S witha motor or engine M. The compressor has a suction line II and pressureline I2 forsupplying compressed air or gas to a conduit I3 that leads tothe point of use and to the intake line ll of the turbine T. The intakeline I4 has a regulating member or valve I5 for controlling thatfraction of the total compressor output that is to be delivered to theturbine to maintain the desired control condition in delivery conduitI3. The turbine exhaust line I6-may include a valve I! to control thedischarge of the exhaust to open air or, alternatively, to return theexhaust gas or air through line I8 to the suction line I I of thecompressor.

It will be apparent that control valves and mechanisms for regulatingthe venting of the surplus compressor output to atmosphere or to theturbine may' be of various known manual and sure is not less than themaximum volume demand, a, delivery conduit into which said pressure lineopens, and regulating means to prevent surging and to maintain a desiredoperating condition at said delivery line when the load demand dropsbelow its maximum value; said regulating means comprising a. conduitbranching from said pressure line, and a regulating valve in said branchconduit for diverting from the delivery conduit that portion of thecompressor output which at the constant operating speed exceeds thevolume corresponding to the desired operating condition at the deliveryline, said regulating means including also power recovery means fordecreasing the load imposed on said power means

